Stem cells strike back: advancements in Alzheimer’s and Parkinson’s disease treatment and modeling efforts from 2019 to 2024

Abstract

This review critically evaluates the current state of stem cell therapy (SCT) for treating and modeling of Alzheimer’s (AD) and Parkinson’s disease (PD). It includes an in-depth analysis of both preclinical and clinical studies, with a particular focus on clinical trials conducted between 2019 and 2024, reflecting recent advancements in the field. Preclinical studies were examined to elucidate the molecular mechanisms underlying SCT and identify developments that could be translated into clinical practice. Within these studies, stem cells, including embryonic stem cells (ESCs), mesenchymal stem cells (MSCs), neural stem cells (NSCs), and induced pluripotent stem cells (iPSCs), have shown high differentiation and proliferation abilities. These properties, along with their capacity to inhibit inflammation, prevent apoptosis, and stimulate angiogenesis, make them promising candidates for treating AD and PD. Over the past 15 years, 76 SCT-based trials have been conducted—27 for AD and 48 for PD—with more than half occurring in the past 5 years. Despite the promise of SCT, the field faces challenges such as ethical concerns regarding the use of ESCs, heterogeneity of isolated cultures, and inconsistent results across preclinical trials. Novel materials and electromagnetic fields (EMFs) offer potential solutions to these issues. While bioengineering approaches can enhance the successful engraftment of transplanted stem cells, EMFs can direct the cells’ migration and differentiation. In conclusion, although significant progress has been made in SCT, ongoing efforts are needed to address existing challenges. Nevertheless, SCT holds considerable promise for the future, offering potential breakthroughs in the treatment of neurodegenerative diseases.

Introduction

Neurodegeneration is a pathological process characterized by progressive neuronal loss and dysfunction, diminishing the function of the central (CNS) or the peripheral nervous system (PNS), occurring as a result of genetic variations, aging, stress, inflammation, traumatic injury, and oxidative stress, as seen in Fig. 1 [1]. Even though they present distinct symptoms, many neurodegenerative disorders are associated with a disruption in protein homeostasis, contributing to neuronal cell death and dysfunction, together with the deposition of inclusion bodies, i.e., aggregates of misfolded and insoluble proteins. As a result, normal cellular mechanisms are disrupted, leading to progressive motor, sensory, and cognitive deterioration [2].

Fig. 1.

Key hallmarks of neurodegeneration in patients with Alzheimer’s and Parkinson’s disorders [3]

Alzheimer’s (AD) and Parkinson’s diseases (PD) are among the most common neurodegenerative disorders, impacting over 65 million people worldwide [4, 5]. AD, the most prevalent cause of dementia worldwide, is typified by the accumulation of amyloid beta plaques (Aβ) and neurofibrillary tangles (NFTs) that trigger synaptic dysfunction and neuronal death [6]. Contrastingly, PD is characterized by the death of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc), which, in turn, causes a range of motor symptoms such as resting tremors and muscle rigidity [7]. On top of the death of DA neurons, PD is also characterized by abnormal protein aggregations, also known as Lewy bodies.

Although both AD and PD are highly prevalent disorders, with their incidence on the rise, no cure exists up to date. Therefore, patients are oftentimes offered pharmaceutical options for disease management, including Levodopa (L-DOPA) and DA agonists for PD, or anti-amyloid drugs for AD. Even though these drugs somewhat effectively manage the disease’s symptoms, they do not represent restorative treatments. Therefore, researchers are increasingly looking into developing and employing novel treatment modalities, one of which is stem cell therapy (SCT). Aside from neurogenesis, transplanted stem cells have also been shown to provide neurotrophic support and regulate remyelination and inflammation [8], thus maintaining the integrity of healthy neuronal circuits and networks via neuroprotective effects [9]. Beyond these effects, stem cells can also support nervous tissue recovery and reduce motor dysfunction by modulating neural and glial cells. Moreover, they can help minimize scar formation and inflammation through anti-inflammatory cytokine release, thereby enhancing neuronal connectivity often impaired in neurodegenerative disorders [10]. Additional regenerative effects include promoting angiogenesis, myelin repair, and immune regulation [11].

Even though both Alzheimer’s and Parkinson’s disorders have been known and studied since the early nineteenth and twentieth centuries, SCT within the context of treatment for patients with AD and PD has not gained clinical significance until 2009, when some of the earliest reported clinical trials took place. Specifically, and by 2024, a total of 76 SCT-based trials, 27 for AD and 48 trials for PD, took place. Around the same number of trials, 7 for AD and 6 for PD, have been completed up to date, with 9 (AD) and 18 (PD) still ongoing. Interestingly, out of the 27 trials employing SCT for patients with AD, a third (9) were started within the last 5 years. This number is only larger for PD, with over a half (25) of the clinical trials taking place over the same 5-year period. Therefore, due to the large volume of SCT-based clinical trials, with almost half (35) of them conducted since 2019, an in-depth exploration of their protocol, results, and effects is needed to provide the reader with an overview of the recent developments within the field.

Overview of stem cell types and characteristics

Stem cells are a type of undifferentiated cell present during embryonic development capable of producing differentiated offspring. Therefore, to qualify under the umbrella term of “stem cells,” two conditions must be satisfied. Firstly, they must possess the ability to self-renew, producing progeny while maintaining their undifferentiated state through symmetric and asymmetric division, a feature that distinguishes them from cancer cells [12]. Secondly, they must be capable of differentiating into specialized adult cell lineages to varying degrees, a characteristic that underlies their classification according to differentiation potency [13]. On top of their differentiation potential, stem cells can also be categorized based on their origin, including, among others, mesenchymal (MSCs), embryonic (ESCs), adult (ASCs), induced pluripotent stem cells (iPSCs), and neural stem cells (NSCs).

Based on their differentiation potential, stem cells can be characterized as either totipotent, pluripotent, multipotent, oligopotent, or unipotent, as shown in Fig. 2. Out of these, totipotent cells demonstrate the greatest differentiation potential, producing all cell types within an organism, including those in both embryonic and extraembryonic tissue. This category of SCs includes the zygote and initial blastomeres. While not as potent, pluripotent stem cells are notable for their capacity to transform into cells of the three germ layers, leading to the development of all cell types in the body [14]. As a result, pluripotent SCs can be obtained from sources of embryonic tissue, including the epiblast, inner cell mass, and primordial germ cells. Next, multipotent stem cells are restricted to the formation of several cell lineages within a single germ layer and can differentiate into cartilage, bone, fat, and muscle, among others [15]. Oligopotent stem cells are more restricted in differential potential since they are limited to multiple (2+) cell lineages in a specific tissue [16]. Lastly, unipotent stem cells have a much more limited differentiation potential and, as such, can only synthesize one type of cell. However, they still retain a high degree of self-renewal, characteristic of stem cells.

Fig. 2.

Stem cell hierarchy depicting the differentiation potential across different potencies, ranging from totipotent to unipotent. Adapted from [12]

On a general level, and depending on the cells’ differentiation potential and its availability in the source tissue, the most employed stem cells in neuroregeneration efforts include multipotent MSCs and NSCs as well as pluripotent ESCs and iPSCs [17]. Mesenchymal stem cells (MSCs) are adult, multipotent stem cells that can be obtained from adipose tissue (AD-MSCs), umbilical cord (UC-MSCs), umbilical cord blood (UCB-MSCs), amniotic fluid (AF-MSCs), oral mucosa (OM-MSCs), Wharton’s Jelly (WJ-MSCs), bone marrow (BM-MSCs), and peripheral blood (PB-MSCs), among others [18]. As such, they can be characterized based on their surface cluster of differentiation (CD) markers, while their fate is determined by their microenvironment [19]. Due to abundant availability, ease of collection, and uncomplicated cultivation, MSCs have gained clinical relevance in recent decades as potential therapeutic tools for addressing a variety of neurological disorders. Their low immunogenicity supports allotransplantation and immunomodulation, with minimal ethical concerns and no oncogenic transformation. However, MSCs can also exhibit significant heterogeneity influenced by donor age and differences in isolation and culture techniques [20]. Next, embryonic stem cells, derived from the inner cell mass of a blastocyst, are pluripotent and can differentiate into all somatic cell types [21]. Moreover, their unlimited self-renewal and plasticity make them promising candidates for therapeutic application. However, since they are typically sourced from in vitro fertilization (IVF)-donated embryos, ESCs use raises ethical concerns, driving interest in alternative generation methods [22]. Offering a more ethical option compared to ESCs, iPSCs are adult cells that have been reprogrammed to regain pluripotency. They originate from adult somatic cells (ASCs) and, through this reprogramming, exhibit characteristics akin to those of ESCs. As such, iPSCs can be used in a variety of cases, including the generation of neural stem cells [23]. Finally, NSCs are a specialized type of stem cell which can self-renew and differentiate into various neuronal and glial cells, playing a crucial role in brain development and repair.

Since neurodegenerative disorders, including PD and AD, are characterized by a progressive death of neurons, novel advancements in the field are needed to slow or stop the disease’s progression [8]. Although still in its early stages, stem cell therapy is showing promise through replacement of diseased neurons, production of neurotrophic factors, or decreasing localized inflammation [24]. While many of the current studies investigating the effects of SCT on neurodegenerative disorders employ different protocols and cell types, most of them point to neuroprotection mediated by brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), or nerve growth factor (NGF) [25], as shown in Fig. 3. Evidence also exists that SCT enhances adult neurogenesis in individuals with neurodegenerative disorders, potentially offsetting the significant neuronal loss seen in these patients [26].

Fig. 3.

Therapeutic effects of stem cells in neurodegenerative disorders [8]

Stem cell therapy for Alzheimer’s disease

Limitations of current treatments and diagnostic approaches

Despite extensive research, Alzheimer’s disease lacks effective treatments, with current management relying on combined pharmacological and behavioral strategies to address symptoms and preserve cognitive function. Currently, there exist two main FDA-approved categories of drugs for the treatment of AD. The first includes acetylcholinesterase (AChE) inhibitors, used within the earlier stages of AD to prevent acetylcholine degradation and enhance neural signalling in memory-related regions [27]. However, they do not clearly slow disease progression and often cause gastrointestinal (GI) side effects like nausea, diarrhea, and vomiting [28]. Moreover, while AChE inhibitors may provide limited relief from symptoms, their effectiveness in slowing AD progression remains unproven, raising concerns about their long-term benefits [29].

The second category of drugs consists of N-methyl-D-aspartate (NMDA) receptor antagonists, such as Memantine, which help regulate calcium levels and reduce glutamate-induced excitotoxicity to improve neuronal signaling in AD patients [27]. Although FDA-approved for moderate to severe AD due to its tolerability and lower receptor affinity, side effects like confusion, dizziness, and urinary tract infections (UTIs) are common [30]. Moreover, due to poor efficacy and adverse reactions, most drugs in this class fail clinical trials [29]. Despite this, NMDA receptor antagonists are occasionally prescribed off-label for mild to moderate cases of AD, raising concerns about their effectiveness in such instances [31].

Due to AD’s complex nature, combination therapy offers greater promise than monotherapy [27]. For example, studies show that combining Memantine with AChE inhibitors improves cognition, behavior, and daily functioning compared to single-drug treatments [32]. Similar results are reported by studies exploring the only currently FDA-approved treatment for moderate to severe Alzheimer’s, combining Donepezil and Memantine, demonstrating increased patient outcomes and improved evaluation scores. Despite this, the treatment is reportedly more expensive and associated with frequent side effects and dosing challenges. In turn, hindered patient adherence and overall drug efficacy—particularly among elderly patients—are noted [27].

To treat the underlying cause of AD, the FDA has also approved two anti-amyloid monoclonal antibody drugs, Aducanumab (2021, discontinued in 2024) and Lecanemab (2023) [27]. These drugs target amyloid fibrils, promoting their clearance via microglial phagocytosis and other mechanisms. While they may reduce plaques and slow cognitive decline, their efficacy and safety remain debated, with side effects like amyloid-related cerebral edemas (ARIA), brain swelling, and seizures, alongside high cost. A summary of the different treatment options, divided across pharmacological and non-pharmacological treatments, can be found in Table 1.

Table 1.

Current treatment approaches for patients with Alzheimer’s and their mechanisms of action

Treatment type		Mechanism of action	Reference
Pharmacological treatments
AChE inhibitors	Rivastigmine	Reversible and non-competitive inhibitor of cholinesterase enzyme, impeding acetylcholine degradation	[27]
	Donepezil	Competitive and noncompetitive, reversible inhibition of acetylcholinesterase, preventing acetylcholine breakdown and enhancing memory	[27]
	Galantamine	Enhances memory and behavior by inhibiting acetylcholinesterase to boost acetylcholine and increasing nicotine receptor sensitivity	[27]
NMDA receptor antagonists	Memantine	Blocks NMDA receptors to impede hyperstimulation and decrease influx of calcium	[27]
Combination Therapy	Donepezil + Memantine	Combination drug encompassing two active components which inhibit acetylcholinesterase and block NMDA receptors	[27]
Anti-amyloid monoclonal antibodies	Aducanumab	Monoclonal antibodies that slow AD progression by targeting fibrillar and oligomeric amyloid plaques for elimination	[33]
	Lecanemab	Monoclonal antibodies that decrease amyloid plaques by attaching to protofibrils	[34]
Orexin receptor antagonists	Suvorexant	Treats AD-related insomnia by antagonizing orexin receptors OX1R and OX2R, blocking orexin A and B binding	[35]
Atypical antipsychotics	Brexpiprazole	Treats dementia-related agitation by modulating dopamine, serotonin, and noradrenergic systems	[36]
Non-pharmacological treatments
Cognitive behavioral therapy		Addresses dementia-related anxiety in AD by adapting psychosocial therapy to cognitive impairments	[37]
Occupational therapy		Improves QoL by supporting independence and daily routine control	[38]
Exercise and physical therapy		Improves coordination, mobility, strength, and balance, while enhancing cognitive function and memory	[39]
Music and light therapy		Music and light therapy actively improves behavior, cognition, and sleep	[40, 41]

Preclinical studies of stem-cell-based treatments for Alzheimer’s and disease modeling efforts

Stem cells and AD are heavily intertwined across various facets, whether regarding disease etiology, modeling, or treatment. Namely, according to Arber et al. mutations in γ-secretase components Presenilin 1 (PSEN1) and Presenilin 2 (PSEN2) are linked to familial forms of AD (fAD), with PSEN1 mutations reducing Notch signaling and disrupting neurogenesis [42]. Unlike neurons which express more β-secretase and amyloid precursor proteins, stem cells show higher γ-secretase and Notch expression. Similarly, disrupted neurogenesis is also observed in iPSC-models of fAD containing mutations in PSEN1 and further corroborated in hippocampal models, all noting a decrease in Notch signaling. As a result, fAD is characterized by the presence of fewer neurons in newborns, leading to premature aging. Such findings were also confirmed by Cao et al. who uncovered a reduction in neurogenesis and a prominence of astrogliosis in AD patients, with a notable decline in the amount of neuroblasts and granule cells [26]. Despite a lack of change in the quantity of NSCs within AD patients in contrast to healthy individuals, the NSCs were inclined to be less active, controlled by Notch and BMP pathways, which gave further depth to the mechanism of AD development. Furthermore, Hurley et al. utilized human induced pluripotent stem cells which were derived from cortical spheroids (hCS) to illustrate how PSEN1 mutations affect neurological development, contributing to fAD advancement [43]. For instance, the researchers portrayed the changes occurring in the development of hCS as a result from the heterozygous PSEN1 L435F mutation that leads to a reduction in mature neurons within the post-mitotic phase, an increase of cell sizes and progenitors, changes to the expression of amyloid beta and alterations in neuronal function.

The use of stem cells has also been significant in providing an effective means of investigating and modeling key mechanisms involved in AD through over 60 in vitro and in vivo studies, most of which utilized iPSCs-derived neurons. Overall, the application of these neurons successfully models key pathological attributes that are characteristic of AD, including aggregation of amyloid beta, modified production of Aβ40 or Aβ42, hyperphosphorylation of TAU, reduction in synapses and their plasticity as well as amplification of oxidative stress [44]. Moreover, iPSC-derived astrocytes have also shown promise in modelling AD-related cellular changes, such as early atrophy and followed by hypertrophy—key events in disease progression. Similar morphological alterations have been observed in oligodendrocytes and microglia [44].

On top of 2D cultures, 3D cerebral organoids are also increasingly being explored as avenues of research into AD pathology, successfully modelling cell interactions, depicting Aβ plaque buildup and phosphorylated tau accumulation [44]. Yet, despite their promise, 3D cerebral organoids also face challenges like poor vascularization, limited synaptic activity, and low reproducibility. While some of these issues can be mitigated in 2D cell cultures, they lack spatial complexity, hindering abnormal protein buildup. On that front, 3D models improve protein aggregation studies through the use of scaffolding materials like hydrogels [45]. However, variability among iPSC lines limits consistency in 3D AD models, while in vivo models, though informative, tend to focus on specific phenotypes and fail to capture the full disease spectrum [46].

Beyond disease modeling, SCs are also being explored as potential therapeutic avenues. For example, several studies have demonstrated that BM-MSCs can aid in AD recovery by replacing malfunctioning neurons with healthy ones, promoting functional recovery [47]. According to Qin et al. BM-MSCs can also trigger autophagy via molecules like Beclin–1, reduce Aβ and tau aggregates, curb neuroinflammation and enhance synapse formation, coupled with outperforming AD-MSCs in cytokine production [48]. Moreover, preclinical Aβ PET imaging also shows that BM-MSC-treated mice display notably better memory function and a reduction in Aβ plaques, when compared to the control group [49].

On top of BM-MSCs, in vivo studies also highlight the promising results underlying AD-MSCs and their exosomes, citing improved learning and memory as well as a reduction in the size of Aβ plaque aggregates within the hippocampus and neocortex [50]. Interestingly, the efficacy of these cells was shown to be further increased following melatonin pretreatment, significantly enhancing cognitive recovery [51]. Aside from direct effects, exosomes obtained from AD-MSCs can also be used as vesicles for drug delivery, as demonstrated by Sheykhhasan et al. [52]. Following COQ10 loading of exosomes, they reported an increase in SOX2 and BDNF levels within the hippocampus, improving cognition more effectively than COQ10 alone.

A major challenge in MSC therapy is poor brain retention, limiting its effectiveness. To address this challenge, Jung et al. integrated iron oxide nanoparticles into WJ-MSCs, boosting therapeutic agent expression [53]. Additionally, since MSCs also act through paracrine signaling, promoting endogenous repair, angiogenesis, and reducing apoptosis, Wang et al. showed that Fe₃O₄,-labeled hUCB-MSCs improved brain delivery and cognitive function in transgenic mice, likely due to increased neuroprotective factor production [54].

While stem cell therapy for AD shows promise in preclinical studies, direct transplantation has yielded limited success. Instead, many researchers are focusing on the application of stem-cell-derived extracellular vesicles (EVs). For instance, the application of hNSC-derived EVs reduced anxiety-like behaviors, decreased Aβ plaque aggregation and microglial activation within 4–6 weeks after intravenous injection via the retro-orbital vein, suggesting protection against synaptic loss [55]. While the study presents promising evidence for the neuroprotective effects of hNSC-derived EVs, it also reflects broader challenges in the field, such as limited age group representation, lack of long-term follow-up, and variable methodological rigor. These issues underscore the importance of developing more standardized and comprehensive preclinical protocols. A detailed overview of the current approaches towards SC-based treatment of Alzheimer’s disease, including existing challenges and outcomes, can be seen in Fig. 4.

Fig. 4.

Stem cell-based therapeutic strategies for Alzheimer’s disease

Case studies and clinical trials

Within the last 5 years, there have been a total of 9 clinical trials exploring the therapeutic effect of stem cells from a multitude of sources for treatment of AD, as seen in Table 2. Of the nine trials, only one has been completed in 2023; however, no results have been published yet. Currently, two clinical trials are actively recruiting participants, while two others are still in the pre-recruitment phase. One of the trials was withdrawn. None of the clinical trials have advanced towards phase 3, with the majority still being in phase 1 or 2.

Table 2.

Clinical trials employing stem cells for treatment of Alzheimer’s disease

Type of SCs	Source	Aim of the study	Results	Country	Trial phase	Clinical trial identifier	Years		Status	Sponsor
							Start	End
MSC	Umbilical cord	Evaluate safety and efficacy of MSC infusions in patients with mild to moderate AD	N/A	USA	1	NCT04040348	2019	2023	Completed	Bernard Baumel
MSC	Human Umbilical cord	Evaluate safety and efficacy of NEUROSTEM® for AD treatment	Deemed safe, well tolerated and feasible. AEs: fever (n = 9), headache (n = 7), nausea (n = 5), vomiting (n = 4), (cleared up in 36 h)	South Korea	1/2a	NCT02054208 NCT03172117	2014	2022	Completed	Medipost Co., Ltd
MSC	Bone Marrow	Evaluate safety and efficacy of Lomecel-B in patients with mild AD	Improvement in cognitive functions and other neurological assessments. Met the end safety point	USA	1	NCT02600130	2016	2021	Completed	Longeveron, Inc
MSC	Adipose tissue	Evaluate the safety profile of HB-adMSCs in participants with clinical diagnosis of AD	N/A	USA	1 & 2	NCT04228666	2020	N/A	Withdrawn	Hope Biosciences
MSC	Adipose tissue	Assess the safety of increasing doses of autologous expanded ADSCs administered via ICV injections to patients with mild to moderate stages of AD	N/A	USA	1	NCT05667649	2023	Est. 2025	Recruiting	Regeneration Biomedical, Inc
MSC	Umbilical cord and amniotic stem cells	Assess the safety, efficacy and tolerability of UC- and AF-MSCs	N/A	USA	1 & 2	NCT04684602	2020	Est. 2030	Recruiting	Thomas Advanced Medical, Llc
MSC	Adipose Tissue	Assess the safety and efficacy of AstroStem (Ad-MSC) in AD patients	N/A	N/A	2	NCT04482413	2023	Est. 2024	Not recruiting yet	Nature Cell Co., Ltd
MSC	Amniotic and umbilical cord tissue	Evaluate safety and efficacy of UC- and AF-MSCs for treatment of various conditions	N/A	N/A	1	NCT03899298	2019	Est. 2024	Not recruiting yet	R3 Stem Cell
MSC	Human Umbilical cord blood	Assess the efficacy of NEUROSTEM® hUCB-MSCs for treatment of AD	N/A	South Korea	1/2a	NCT04954534	2021	Est. 2022	Unknown	Samsung Medical Center
MSC	Adipose tissue	Evaluate the safety and efficacy of MSC-derived exosomes in treating mild to moderate AD	N/A	China	1 & 2	NCT04388982	2020	Est. 2022	Unknown	Ruijin Hospital
MSC	Adipose tissue	Evaluate the safety and efficacy of SC-isolates extracted from microvasculature to treat neurological disorders.	N/A	USA	N/A	NCT03297177	2020	Est. 2023	Unknown	Regeneris Medical

In 2019, a phase 1 open-label clinical trial began investigating the tolerability, safety profile, and potential effectiveness of treating AD patients with mild to moderate symptoms using infusions of homologous human mesenchymal stem cells (hMSCs) [56]. This study involved 6 participants aged 50–85 with a Mini-Mental Status Examination (MMSE) score of 20–26, confirmed amyloid presence, and meeting National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer’s Disease and Related Disorders Association (NINCDS-ADRDA) criteria. AChE inhibitor users required stable doses for 3 + months, and female participants had to be postmenopausal, sterilized, or infertile. During the clinical trial, the treatment group received four overall doses (1 per 13 weeks) of the hMSC infusions, with about 100 million allogeneic hUC-MSCs dispensed intravenously. This clinical trial was completed in 2023; however, no results have been published yet.

On the other hand, in 2021, the results were published for a phase 1/2a double-blind clinical trial investigating the safety and efficacy of administering UC-MSCs intraventricularly in patients with mild to moderate AD [57]. After this, a follow-up clinical trial was enacted to examine the long-term safety and efficacy in patients who completed the initial trial within a 36-month period post preliminary administration. The participants were Korean, aged 50–85, with probable AD or mild cognitive decline per NINCDS-ADRDA criteria, a Korean-Mini-Mental Status Examination (K-MMSE) score of 18–26, and positive amyloid PET scans or mild neurodegeneration on MRI [57]. During the course of the clinical trial, NEUROSTEM® system’s Ommaya reservoir, an intraventricular catheter system, was inserted into the right lateral ventricle of participants, 4 weeks before MSC administration. Of all the participants, six were administered with a high dose of hUBC-MSCs (3.0 × 10⁷ cells/2 mL), whilst three patients were given the low dose (1.0 × 10⁷ cells/2 mL). Across 4-week intervals, MSCs injections were repeated three times in all nine patients. The participants were then monitored for up to 12 weeks post initial injection, and then an extra 36 months in a follow-up study. As for the results of this trial, the MSCs were deemed to be safe, well tolerated, and feasible. Interestingly, the most prevalent adverse effect (AE) was fever, present in all 9 patients, followed by 7 patients exhibiting headaches, 5 having nausea, and 4 reporting vomiting. However, all these AEs cleared up within 36 h. Meanwhile, despite three serious adverse events identified in two patients, no dose-limiting toxicities were observed. Lastly, 5 of the 9 patients continued on to the follow-up study, where there were no additional serious AEs observed [57]. Despite reporting only transient adverse effects and no dose-limiting toxicities, the small sample size and limited follow-up participation in the aforementioned study highlight the need for larger, long-term investigations to confirm these findings.

Similarly, in 2023, the results for another phase 1 clinical trial investigating the efficacy and safety profile of Lomecel-B, which is an exogenous medicinal signaling cell, for treatment of mild AD were reported. Specifically, the participants in this Phase 1, double-blind randomized trial were divided into groups and received a single infusion of either a high dose, a low dose of Lomecel-B, or a placebo [58]. As for the results of this trial, it met the specified primary safety endpoint. Moreover, in comparison with the placebo, a significant improvement in cognitive functions and other neurological parameters was observed in the treatment group, warranting further investigation in larger cohorts.

Aside from completed clinical trials, recent years have also seen the withdrawal of some. Specifically, in 2020, an open-label phase 1/2 clinical trial was planned to evaluate the safety and efficacy of four intravenous infusions of Hope Biosciences adipose-derived mesenchymal stem cells (HB-adMSCs) [59]. Each infusion, containing a dose of 2 × 10⁸ HB-adMSCs, was scheduled to be administered at weeks 0, 2, 6, and 8 to potentially treat patients diagnosed with AD [59]. The study aimed to quantify AE prevalence as the primary endpoint and assess HB-adMSCs’ effects on AD inflammation and amyloid buildup (Aβ40, Aβ42) as the secondary endpoint. Cognitive changes were to be evaluated via MMSE, ADRQL, and NMI. Participants, aged 50–85, with early-stage probable AD and positive amyloid PET scans, were eligible. Despite aiming to recruit 24 patients, the study failed to recruit any, leading to its withdrawal due to the COVID-19 pandemic.

Currently, there are four ongoing clinical trials employing stem cells for the treatment of AD. The first two clinical trials are currently in the recruitment phase, focusing on evaluating the safety and efficacy of their respective stem cell treatments on distinct sample groups. One of these trials is within phase 1 and aims to assess the safety of increasing doses of autologous expanded ADSCs administered via ICV injections to patients with mild to moderate AD who are less responsive to conventional treatments [60]. As for the second clinical trial, it is a phase 1/2 multi-arm and multi-site interventional trial beginning in 2020, examining safety profiles, efficacy, and tolerability of stem cell application within treating several chronic and acute disorders such as neurodegenerative disorders, including AD [61].

Regarding the two other clinical trials, they are yet to begin with participant recruitment. The first clinical trial will be carried out to investigate the safety and efficacy of AstroStem, a novel treatment modality comprising AD-MSCs, in AD patients using a double-blind randomized control test. This phase 2b trial plans to enroll 100 AD patients diagnosed within a year, providing intravenous treatment every 4 weeks from week 1 to 36, followed by evaluations at weeks 44 and 52 [62]. As for the second study, it is a phase 1 trial exploring the safety and efficacy of MSCs from umbilical cord and amniotic tissue for various conditions. For AD, treatment involves IV and intranasal administration, with data collected over 10 years and dementia being assessed via MMSE in 500 participants [63].

As of yet, there also exist three clinical trials whose status remains unknown. The first study is a phase 1/2a clinical trial investigating the efficacy of hUCB-MSCs in treating patients aged 50–85 diagnosed with AD by administering a high dose of hUCB-MSCs (3 × 10⁷ cells/2 mL), to be repeated three times across 4-week intervals. The status of this clinical trial is still unknown (N/A) [64]. The next study is an open label phase 1/2 clinical trial inspecting the safety and efficacy of treating mild to moderate dementia (AD), by utilizing exosomes extracted from adipose MSCs [65]. Lastly, the final clinical trial is evaluating the safety and efficacy of extracting stem cell isolates from microvasculature to treat non-cancerous diseases and neurological conditions, such as AD [66]. A detailed overview of all clinical trials can be found in Table 2.

Stem cell therapy for Parkinson’s disease

Limitations of current treatments and diagnostic approaches

Current PD treatments mainly focus on symptom management, categorized into pharmacological (medications), surgical (e.g., deep brain stimulation for advanced cases), and non-pharmacological approaches (physical therapy, occupational therapy, and lifestyle changes), as seen in Table 3.

Table 3.

Current treatment approaches for patients with Alzheimer’s and their mechanisms of action

Treatment type		Mechanism of action	Reference
Pharmacological treatments
Levodopa	Sinemet Madopar Stalevo Duodopa Corbilta Stastravi	A dopamine precursor converted in the brain to counteract dopamine and neuron loss	[67]
Dopamine agonists	Pramipexole Ropinirole Rotigotine Apomorphine Cabergoline Bromocriptine	Can mimic the function of dopamine and thereby counteract the loss of dopamine and DA neurons	[68]
MAO-B inhibitors	Eldepryl Zelapar Azilect Xadago	Mainly block MAO-B to inhibit dopamine breakdown in the brain, with added neuroprotective effects	[68]
COMT inhibitors	Comtess Comtan Tasmar Ongentys	Inhibit COMT to prolong dopamine action, compensating for its loss due to DA neuron destruction in PD	[68]
Anticholinergics	Trihexyphenidyl Biperiden Procyclidine	Mainly used to manage tremors in PD patients. Anticholinergics block the action of the neurotransmitter acetylcholine and thereby reduce tremors	[67]
Amantadine	Symmetrel Mantadix	Increases the release of dopamine and inhibits the uptake of acetylcholine, helping to manage tremors	[67]
Surgical treatments
Deep brain stimulation (DBS)		DBS is a procedure where small electrodes are implanted in specific areas of the brain to improve motor functions	[69]
Lesioning surgeries		Lesioning surgery creates targeted brain lesions to alleviate symptoms when medications fail, focusing on areas like the thalamus, globus pallidus internus, or subthalamic nucleus	[70]
Non-pharmacological treatments
Physical therapy		Physical therapy manages PD symptoms, improving patients’ mobility and balance	[71]
Occupational therapy		Occupational therapy for PD patients focuses on maintaining their independence
Speech therapy		Motor and cognitive changes in PD affect speech, which speech therapy aims to address	[72]
Exercise		An exercise routine helps keep the patients active	[71]
Transcranial magnetic stimulation (TMS)		TMS is a non-invasive treatment option in which specific parts of the brain can be stimulated	[73]

Levodopa is the most common treatment for PD, crossing the BBB and converting to dopamine to offset neuron loss [68]. Similar to Levodopa, dopamine agonists counteract dopamine loss, while monoamine oxidase B (MAO-B) inhibitors increase dopamine by blocking its breakdown. Similarly, catechol-O-methyltransferase (COMT) inhibitors, like MAO-B inhibitors, target catecholamines to extend Levodopa’s half-life, stabilize dopamine levels, and ensure a consistent dopamine cycle. In contrast, anticholinergics and amantadine restore dopamine-acetylcholine balance, effectively reducing tremors and motor symptoms in PD.

When medications are ineffective, invasive procedures like deep brain stimulation (DBS) or lesioning surgery may be considered. DBS involves implanting electrodes in specific brain regions to improve motor symptoms and reduce medication use, though risks include bleeding and infection. In addition to pharmacological and surgical interventions, it is crucial to consider non-pharmaceutical approaches to ensure the best possible care and QoL. These primarily include physical, occupational, and behavioral therapies, at times supplemented with novel approaches such as transcranial magnetic stimulation (TMS)—a procedure which involves sending short magnetic pulses to specific areas of the brain to improve motor function.

Preclinical studies of stem-cell based treatments for Parkinson’s and disease modeling efforts

iPSCs and 3D brain organoids have significantly advanced our understanding of PD pathophysiology and assisted in drug development by allowing us to replicate the specific mutations commonly found in familial PD (fPD), including those in α-synuclein [74, 75]. Similar advancements can also be found in in vivo studies on transgenic rats and mice with SNCA mutations, although they often fail to clearly exhibit dopaminergic neurodegeneration [76].

One of the most prominent approaches within the field is the use of patient-specific iPSCs, which exhibit susceptibility to stressors causing cell death [77], abnormalities in mitochondrial dynamics and function [78] and aberrant neuronal morphology in long-term culture [79]. In addition to this, PD-derived neuronal precursor cells also show an increase in Hox gene transcription, providing new insights into the disease physiology and novel therapeutic targets [80]. Other studies reported that PRKN mutation-carrying iPSC-derived DA neurons exhibited accumulation of α-synuclein, with Parkin expression rescuing the phenotypes [81].

Efforts to model PD using stem cell-derived neuronal cultures were also performed by Oosterven et al. wherein successful direction of mouse and human iPSC-derived dopamine progenitors into SN-like neurons by using WNT inhibitors or SOX6 expression was reported [82]. Therefore, while traditional in vitro models have advanced our understanding of PD, emerging technologies like iPSCs and organoids offer more promising avenues for studying its pathogenesis [83]. Ultimately, as many of these studies have shown, a key advantage of iPSCs is their retention of the donor’s genetic background, allowing for a more realistic representation of the disease pathophysiology [84]. Nonetheless, key limitations persist, including variability in differentiation protocols and the heterogeneity of resulting neuronal populations.

Since traditional cell cultures often fall short in accurately replicating the cellular microenvironment, recent years have witnessed a significant increase in the use of organoids for disease modeling. When it comes to PD, iPSCs from patients and healthy donors are used to generate human midbrain organoids (hMOs) with layered structures resembling the in vivo midbrain floor plate. Following single-cell transcriptomics analysis, Patikas et al. found that these hMOs exhibit greater cellular heterogeneity compared to the monolayer culture and a strong upregulation in hypoxia inducible factor 1α (HIF1α) in SNCA-3 × neurons [85], which has been associated with PD and brain aging [86]. Exploration into iPSCs ability of differentiation into DA neurons was also performed by Cardo et al. wherein the authors successfully demonstrated that CRISPR/Cas9-assisted genome editing can knock-in Cre and silent blue fluorescent protein (BFP) expression, thereby enabling precise monitoring of the differentiation process [87]. While these models demonstrate greater cellular complexity and enable precise tracking of dopaminergic differentiation, their limited maturation and lack of aging-related features continue to pose challenges for faithfully recapitulating long-term disease progression.

On top of being useful in modeling disease pathophysiology, organoids can also be employed for testing of novel treatment approaches. One of these efforts was spearheaded by Mendez-Pinheiro et al. wherein hMOs from three different hvNESCs lines were used to investigate the effects of BM-MSCs’ secretome on PD phenotypes [88]. The hMOs featured a central stem cell niche, with asymmetrically distributed dopaminergic neurons, and expressed midbrain floorplate markers FOXA2 and EN1. Additionally, the presence of dopamine and co-expression of GIRK2 with tyrosine hydroxylase (TH) indicated the presence of A9 subtype midbrain dopaminergic neurons, forming a complex neuronal network. Following the application of BM-MSCs, the researchers demonstrated protective effects of BM-MSCs’ secretome, maintaining TH + dopaminergic neurons in hMOs exposed to 6-OHDA. Similar results were reported in 6-OHDA-lesioned mice, where improvements in motor skills were observed.

Aside from being used as modalities for testing of novel treatment approaches, hiPSC-based organoids are also being employed as grafts, showing significant benefits for PD treatment. In a study conducted by Zhang et al. severe combined immunodeficient (SCID) mice with 6-hydroxydopamine (6-OHDA) lesions were utilized to evaluate the efficacy of hMOs. These hMOs have demonstrated the capability to mature into dopaminergic (DA) neurons expressing key markers such as FOXA2, NURR1, TH, and G-protein inwardly rectifying potassium channel 2 (GIRK2) [89]. Post-transplantation, hMOs exhibited normal electrophysiological activity and improved motor function in a PD mouse model. They also modulated striatal inflammation by reducing proinflammatory and increasing anti-inflammatory cytokines, likely through synapse formation with endogenous medium spiny neurons (MSNs). These MSNs subsequently project to the prefrontal cortex (PFC) and hypothalamus (HY), facilitating the observed functional improvements.

The benefits of iPSCs were also investigated by Guo et al. who demonstrated that rat iPSCs-derived primitive neuroepithelial cells (RiPSCs-iNECs) could be induced to form neurospheres and could, consequently, differentiate into neurons and astrocytes in vitro [90]. The iPSCs were induced from adult skin fibroblasts from rats using six reprogramming factors. When transplanted into the substantia nigra of 6-OHDA-lesioned PD rats, RiPSCs-iNECs survived for at least 150 days, differentiated into various types of functional neurocytes, and led to significant behavioral recovery in the rats. Similarly, Doi et al. isolated peripheral blood cells and reprogrammed them to iPSCs using episomal plasmid vectors expressing different specific transcription factors like SOX2, KLF4, and OCT3/4, among others [91]. The iPSCs were then differentiated into dopaminergic progenitors (DAPs). When transplanted into the striatum of 6-OHDA-lesioned rats, the DAPs led to behavioral improvements in the animals. Since the findings showed no tumorigenicity or toxicity of the cells, the clinical trial started in 2018 [91]. These findings were also echoed by Liang et al. who showed that TREM2 induces iPSCs’ differentiation into dopaminergic neurons [92]. For the study, 36 C57BL/6 mice with an established PD model, induced by 6-OHDA injections in the right striatum, were used. The mice were stereotaxically injected with normal saline, untreated, and transfected iPSCs. The results demonstrated that TREM2 overexpression may potentially promote direct differentiation of iPSCs into specific dopaminergic neurons and also enhance neuronal repair in the PD mouse model [92]. Likewise, Guo et al. used iPSCs reprogrammed from miniature-swine dermal fibroblasts [93]. The iPSCs were further differentiated into GABA progenitors (MiPSC-iGABAPs) and subsequently transplanted into the right medial forebrain bundle of rats with 6-OHDA-induced lesions [93]. For this study, 27 PD and 9 healthy control rats were used, demonstrating an efficient differentiation procedure of GABA progenitors from iPSCs. Additionally, the authors have also shown that the transplantation of the MiPSC-iGABAPs significantly improved the motor function in PD rats. Taken together, these studies underscore the therapeutic potential of iPSC-derived neural progenitors in PD models, yet the variability in reprogramming methods reported in the aforementioned studies, limited scalability, and unresolved concerns around long-term safety and functional integration highlight the need for more standardized and clinically validated approaches.

Since one of the main challenges in the translation of preclinical studies into clinics is ensuring successful engraftment of transplanted SCs, Kriks et al. are working on developing a new floor-plate based protocol for differentiating hiPSCs into midbrain DA neurons [94]. Following the study, the authors have shown long-term survival and functional integration in animal models. To confirm these findings, the floorplate-derived dopaminergic neurons were transplanted into a PD model of rats, lesioned using 6-OHDA. On top of behavioral improvements in rats, the authors demonstrated the scalability of their method by transplanting cells into parkinsonian monkeys, showing excellent cell survival and function across different animal models, proposing promising development of cell-based therapies for PD.

On top of iPSCs, other studies have opted for using mice or human ESCs for studying their effects in mice models of PD. For example, Xiong et al. have used hESC-derived midbrain dopaminergic (mDA) or cortical glutamate neurons that were transplanted into the SN or the striatum of a mouse PD model. The mice used in this study were adult SCID mice, with 6-OHDA injected into the left SN [95]. Following treatment application, the researchers observed thorough graft integration with the host circuit. Overall, the study highlighted that the functional success of transplants into the brain primarily relies on the intrinsic properties of the grafted neurons, emphasizing the importance of using precisely characterized cells. Similarly, Edwards et al. have used ESCs derived from a 129/Sv mouse and MSCs from a C57BL/6 in order to derive neural progenitors [96]. This study used an MPTP-injected mouse model of PD to test the efficacy of peripheral injections of ESC- and MSC-derived neural progenitors. The main goal was to assess motor function post-treatment. Results showed a significant reduction in astrocyte and microglia activation, suggesting decreased neuroinflammation, with no observed tumor development, indicating safety.

Aside from pluripotent iPSCs and ESCs, other research is looking into the effects of MSC-derived secretome [88] and exosomes [97], as well as investigating BM-MSCs ability to be reprogrammed into induced neuronal stem cells (iNSCs) [98]. Following reprogramming of BM-MSCs, the iNSC-derived DA neuron precursors have been shown to be safe and efficient in a PD mouse model [98]. Interestingly, these iNSCs have demonstrated all the characteristics of normal NSCs, boasting the ability to differentiate into astrocytes, neurons, and oligodendrocytes. In order to optimize the engraftment of transplanted cells, other researchers are looking into employing MNPs as carriers for targeted delivery to the SN. Following transplantation into a 6-OHDA-induced PD mouse model of 15 male C57BL/6N mice, an increased distribution of MNP-labeled hAD-MSCs in the SN was observed, coupled with improved motor function and recovery of nigrostriatal DA neurons [99].

Continuing their work on MSC-based treatments of PD, Sun et al. have developed a protocol for hAD-MSCs isolation and induction of the expression of GDNF using the lentiviral vector (LV) system [100]. Sixteen 6-week-old male C57BL/6 J mice were used to study the effect of GDNF on the viability and survival rate of hAD-MSCs. In this study, a unilateral 6-OHDA lesion was used to establish a PD mouse model. Injection of GDNF-loaded hAD-MSCs into the left striatum enhanced cell survival and differentiation while also improving motor function in the lesioned mice. The 6-OHDA mouse model of PD was also employed by Li et al. who studied the effects of VEGF-expressing human adipose-derived mesenchymal stem cells (hAMSC-VEGF189-GFP) on male 8–9-week-old C57BL/6 J mice with PD [101]. Since previous studies had shown that VEGF plays an important role in pain modulation, the goal of this research was to investigate the therapeutic effects of VEGF-expressing hAD-MSCs on PD-related pain and explore its underlying mechanisms. Following transplantation, a slight reduction in transient receptor potential vanilloid 1 (TRPV1) expression in trigeminal ganglia (TG) neurons was observed. This indicates that hAMSC-VEGF189-GFP can reduce TRPV1 sensitization in central terminal and peripheral sensory neurons, leading to a decrease in PD-related pain [101]. Ultimately, these targeted approaches using genetically modified MSCs not only appear to address motor deficits but also extend therapeutic potential to non-motor symptoms like pain, a welcome addition within the field. While current studies highlight promising therapeutic avenues, future research should focus on validating long-term safety and efficacy through larger cohorts, while also examining the functional stability of transplanted cells, their integration with host neural circuits, and any potential off-target or neuroimmune effects to ensure clinical translatability.

Aside from 6-OHDA models of PD, other research reported employing transgenic models, like MitoPark, as shown by Chan et al. [102]. Specifically, following the isolation of hAD-MSCs from healthy donors, the cells were validated based on their surface markers. Next, exosomes were isolated from the culture medium and subsequently administered intravenously to MitoPark mice, a transgenic Parkinson’s disease model with a conditional knockout of mitochondrial transcription factor A (MTFA) in dopaminergic neurons. Following treatment administration, the hADSC-derived exosomes demonstrated neuroprotective properties due to their anti-inflammatory effects and prevented the progression of motor dysfunction. Given their low immunogenicity and potential for large-scale production, they represent a promising therapy for PD and other neurodegenerative diseases.

Since rats have been shown to exhibit more consistent responses to some treatments than mice [103], others are opting for the use of rat models of PD to research the effects of SCT. Namely, Simorgh et al. used 30 adult male Wistar rats following intrastriatal 6-OHDA injections in the right striatum [104]. In this study, they aimed to evaluate the efficacy of intranasal delivery of olfactory ecto-mesenchymal stem cells (OE-MSCs) located in the lamina propria of the olfactory mucosa. The study successfully demonstrated OE-MSCs’ delivery, with transplanted cells detected in various brain regions. Moreover, the authors also noted significant improvements in behavior and dopaminergic functions in PD rats, suggesting that intranasal delivery of OE-MSCs may be a promising and minimally invasive alternative in PD treatment [104].

Although delivering neurotrophic factors via genetically modified MSCs shows promise for neuroprotection in neurodegenerative diseases, poor post-transplant survival and host immune response, such as microglial activation and astrocyte recruitment, remain significant barriers to clinical translation. To address this, researchers have begun incorporating biomaterials as a supportive matrix, enhancing cell viability and modulating host response. Specifically, Hoban et al. have shown that the delivery of GDNF-overexpressing MSCs (GDNF-MSCs) to the intact rat striatum elicited reduced microglial and astrocytic recruitment, suggesting improved biocompatibility [105]. Although this study was performed on healthy rats, it is an important step towards ensuring successful engraftment of transplanted cells in PD patients as well. Similarly, and in order to test its efficacy in PD models, Santaella et al. turned to transplanting collagen-encapsulated hUC-MSCs into the striatum of male hemi-Parkinsonian Sprague–Dawley rats [106]. The study consisted of 40 rats who received a unilateral intramedial forebrain bundle lesion utilizing 6-OHDA. Although widespread changes in the host tissue were expected, like those taking place in the study by Hoban et al. Santaella et al. reported no difference in specific protein expression between treatment and control groups. They hypothesized that the lack of visible differences might be due to the effects being too localized and subtle to detect.

Finally, Marei et al. used OB-NSCs genetically engineered to express hNGF and GFP to track their effects once transplanted into the striatum of 68 male 6-OHDA Parkinsonian Wistar rats [107]. Following transplantation, differentiation of these cells into oligodendrocyte-like, neuron-like, and astrocyte-like lineages was observed, yielding improvement in motor function. Similar results were reported by Alizadeh et al. wherein differentiation of OB-NSCs into DA-like neurons was observed in vitro [108]. Besides rats, other researchers are also employing mice models of PD, including those seen in the work of Li et al. [109]. In this study, NSCs were derived from the bone marrow of male C57BL/6 GFP transgenic mice. To model PD, 60 male C57BL/6 mice (10–12 weeks old) were injected with MPTP and kept under pathogen-free conditions. The goal of the study was to investigate the potential protective effects of Fasudil in conjunction with NSCs, revealing that NSC transplantation alone might not be sufficient for nerve regeneration. However, the addition of Fasudil has been shown to significantly enhance the therapeutic potential of NSC transplantation, suggesting a synergistic effect that improves treatment outcomes. The MPTP model of PD was also employed by Altarche-Xifro et al. who investigated the effects of hematopoietic stem and progenitor cells (HSPCs) on 8- to 12-week-old male mice [110]. Following treatment administration, the authors have shown that HSPCs can fuse with TH + cells but not with GAD + or GABA + cells. Additionally, it appeared that HSPC transplantation may protect against dopaminergic neuron loss. Although a significant decrease in DA neuron loss was reported, a drastic decline in these cells occurred just 1 week after transplantation due to the immune response. Therefore, despite some encouraging outcomes, NSC transplantation has generally yielded inconsistent results across PD models, with limited evidence of sustained nerve regeneration—likely reflecting variability in cell sourcing, host immune responses, and the complex interplay between transplanted cells and the diseased microenvironment. To improve consistency and therapeutic efficacy, steps should be taken towards standardizing cell sourcing protocols, preconditioning NSCs with MSC-derived secretomes to enhance survival and differentiation, and employing biomaterial scaffolds like electroactive polymer-based scaffolds that better replicate the electrical and structural cues of the pathological niche [111], thereby supporting NSC survival and guiding neural differentiation.

Since isolation of NSCs can sometimes prove to be infeasible due to the potential complications of the procedure and their low availability, other researchers are looking into reprogramming peripheral blood mononuclear cells (PBMCs) isolated from donors’ peripheral venous blood into induced iNSCs [112]. Specifically, this research utilized adult male SCID-beige and C57BL/6 mice to create unilateral PD models by injecting 6-OHDA into the striatum of the right hemisphere. To compare the effects of iNSCs with native NSCs, fetal NSCs were isolated from aborted fetal brain tissue. The findings suggested that autologous dopaminergic cells might be preferable to allogeneic dopaminergic precursors due to their ability to avoid immune recognition issues. Additionally, iNSCs derived from PBMCs showed greater plasticity than native NSCs, demonstrating the potential to differentiate into various types of neurons. However, the study also raised safety concerns, noting the presence of some mutations in the iNSCs.

In addition to direct stem cell transplantation into target tissues, stem cell-derived EVs, such as exosomes, are emerging as novel therapeutic approaches. This is primarily because microRNAs (miRNAs), known for their neurogenic and neuroprotective properties, present a promising therapeutic candidate for PD. However, the challenge remains in achieving efficient delivery of miRNAs to the brain. Using exosomes as delivery vehicles may help overcome this obstacle, enhancing the therapeutic potential of miRNAs in PD treatment. Using exosomes (Exos) derived from hUC-MSCs, Chen et al. have demonstrated their neuroprotective effects on DA neurons in 6-OHDA rats [102]. Following hUC-MSC-Exo delivery, increased cell viability and striatal DA levels, together with a decreased expression of apoptosis-related proteins, were observed. Similar results were reported by He et al. including improvements in motor function deficits and reduced α-synuclein aggregation in male C57BL/6 PD mice [113]. In this study, phenotypically uniform trophoblast stage-derived mesenchymal stem cells T-MSCs and their exosomes (T-MSCs-Exo) were investigated for their potential to protect DA neurons in PD models. On top of demonstrating T-MSCs-Exo’s potential, the study also highlighted that exosomal miRNAs, such as miR-100-5p, may play an important role in the regulation of oxidative stress and neuron protection in PD. Although the results of the study showed to be promising, they acknowledged limitations in the translation of the in vivo studies to clinical trials due to the difference in the human and mouse physiology.

The neuroprotective effects of miRNAs were also reported by Esteves et al. wherein miR-124-3p-loaded umbilical cord blood mononuclear cell-derived small extracellular vesicles (sEVs) were transplanted into 6-OHDA mice [114]. The results of the study demonstrated that miR-124-3p sEVs safeguard DA neurons from 6-OHDA-induced damage in both in vitro and in vivo models, ultimately leading to complete motor function recovery in vivo. Furthermore, and in order to optimize exosomes as delivery vehicles, Kojima et al. have developed a set of devices, called EXOtic devices, that enable efficient and customizable production of designer exosomes in engineered mammalian cells [115]. These EXOtic devices can deliver messenger RNA (mRNA) to target cells, including those in the brain. Specifically, the study involved engineered cells implanted in eight C57BL/6 J female mice, successfully delivering cargo mRNA to the brain. In turn, the mRNA reduced neurotoxicity and neuroinflammation in both in vitro and in vivo PD models. A detailed overview of the current approaches towards SC-based treatment of Parkinson’s disease, including common strategies and challenges, can be seen in Fig. 5.

Fig. 5.

Stem cell-based therapeutic strategies for Parkinson’s disease

Case studies and clinical trials

Over the past 5 years, from the beginning of 2019 until now, there have been 25 studies focused on PD and stem cell therapies, as seen in Table 4. Of these, four studies have been completed, with results available for one. Additionally, seven studies are currently recruiting participants. The first of the completed studies was an interventional study to evaluate the effectiveness of platelet-rich plasma (PRP) and SCT in the treatment of PD [116]. The study, conducted from 2019 to 2022, involved 30 participants aged from 30 to 50 who had been diagnosed with PD within the last 1–3 years. To be eligible, participants needed to be on stable therapy with DA agents and/or DBS parameters. This open-label study featured one experimental group, where patients received three PRP sessions. Next, a phase 2a randomized placebo-controlled trial, taking place from 2020 until 2023, aimed to find the safest and most effective number of allogeneic BM-MSC infusions to slow PD progression [117]. It included 45 patients aged 50 to 79, diagnosed with PD for 3–10 years, showing at least two cardinal signs, including bradykinesia, and a strong response to dopaminergic therapy. The study had three arms: one group received MSCs and placebo, the second only MSCs, and the third only placebo.

Table 4.

Clinical trials employing stem cells for treatment of Parkinson’s disease

Type of SCs	Source	Aim of the study	Results	Country	Trial phase	Clinical trial identifier	Years		Status	Sponsor
							Start	End
MSC	Adipose tissue	Assess the safety of autologous MitoCell administrations and explore the efficacy	N/A	N/A	1	NCT05094011	2025	2027	Not yet recruiting	Taiwan Mitochondrion Applied Technology Co., Ltd
MSC	Bone Marrow	Find the safest and most effective number of repeat doses of allogeneic bone marrow-derived mesenchymal stem cell (MSC) infusions to slow the progression of PD	N/A	USA	2	NCT04506073	2020	2023	Completed	University of Texas Health Science Center
hDAPCs	N/A	Assess the safety, tolerability, and preliminary efficacy of a single injection of human dopaminergic progenitor cells (hDAPCs)	N/A	China	1/2	NCT06167681	2024	2029	Recruiting	iRegene Therapeutics Co., Ltd
A9-DPCs	ESC	Evaluate long term safety of allogeneic embryonic stem cell derived A9 dopamine progenitor cell (A9-DPCs)	N/A	South Korea	N/A	NCT06477744	2023	2029	Active, Not Recruiting	S.Biomedics Co., Ltd
hESC-derived DA cells	Epithelial stem cells	Assess the safety and efficacy of STEM-PD in people with PD	N/A	Sweden	1	NCT05635409	2022	2027	Recruiting	Region Skane
hAESCs	Placental amnion	Investigate the safety, tolerability and efficacy of hAESCs for PD	N/A	China	1	NCT05691114	2023	2026	Recruiting	Shanghai East Hospital
hAESCs	Placental amnion	Investigate the safety and efficacy of stereotactic transplantation of hAESCs for PD	N/A	China	1	NCT04414813	2020	2023	Completed	Shanghai East Hospital
NSCs	iPSCs	Investigate the safety and efficacy of iPS-NSC for PD	N/A	N/A	1	NCT03815071	2019	2021	Unknown	Allife Medical Science and Technology Co., Ltd
A9-DPCs	ESCs	Identify the maximum tolerable dose and assess safety and efficacy of A9-DPC therapy for PD patients	N/A	South Korea	1/2	NCT05887466	2023	2026	Active, Not recruiting	Allife Medical Science and Technology Co., Ltd
NSCs	PBMCs	Investigate safety and efficacy of autologous induced neural stem cell-derived dopaminergic precursor cells in PD	N/A	China	1	NCT05901818	2023	2026	Recruiting	Xuanwu Hospital
MSCs	Umbilical cord	Monitor possible adverse events or complications of intravenous infusions of cultured allogeneic adult umbilical cord derived MSC	N/A	Antigua & Barbuda	1	NCT05152394	2022	2026	Not yet Recruiting	Foundation for Orthopaedics and Regenerative Medicine
iPSCs-derived DA neural progenitors	N/A	Investigate safety and efficacy of transplantation of human iPSC-derived dopamine neural progenitors for PD	N/A	USA	1	NCT06482268	2024	2028	Recruiting	University of California, San Diego
Pluripotent stem cells	Adipose tissue	Investigate safety and efficacy of allogeneic pluripotent stem cells in patients with PD	N/A	Costa Rica	1/2	NCT06141317	2023	2024	Active not yet recruiting	ClusterXStem-Costa Rica
hiPSCs	N/A	Investigate safety and efficacy of stem cell investigational drugs	N/A	China	N/A	NCT06145711	2023	2025	Not yet recruiting	Shanghai East Hospital
iPSCs	Skin	Investigatethe safety and tolerability of injecting ANPD001 cells	N/A	USA	1	NCT06344026	2024	2030	Enrolling by invitation	Aspen Neuroscience
MSCs	Adipose tissue	Investigate the safety and efficacy of multiple HB-adMSCs for the treatment of PD	Changes in MDS-UPDRS, CBC, and Neuro-QoL following HB-adMSC treatment	USA	2	NCT04928287	2021	2023	Completed	Hope Biosciences
Natural stem cell mobilizers	N/A	Investigate safety and efficacy of a new food supplement composed of natural stem cell mobilizers	N/A	Spain	N/A	NCT05699694	2023	2025	Recruiting	Sociedad Española de Medicina Regenerativa y Terapia Celular
Epithelial Stem Cells	Amnion	Investigate the safety of multiple treatments and also the effectiveness of hAESC	N/A	China	1	NCT05435755	2022	2023	Not yet recruiting	Shanghai East Hospital
ESCs	N/A	Investigate the safety and clinical outcome of patients who had previously received BRT-DA01	N/A	USA	1	NCT05897957	2023	2027	Enrolling by invitation	BlueRock Therapeutics
ESCs	N/A	Investigate safety of injections of nerve cells in the brain	N/A	USA	1	NCT04802733	2021	2024	Unknown	BlueRock Therapeutics
MSCs	Adipose tissue	Investigate safety and efficacy of multiple allogenic HB-adMSC for treatment of PD patients	N/A	USA	2	NCT04995081	2021	2026	Recruiting	Hope Biosciences
Embryonic-like stem cells	Peripheral blood	Investigate the improvement in The Unified PD Rating Scale	N/A	Pakistan	N/A	NCT06142981	2019	2022	Completed	Fatima Jinnah Medical University
ASCs	CNS	Investigate safety and efficacy of the transplantation of neural stem cell-derived neurons for PD treatment	N/A	N/A	1/2	NCT03309514	2022	2024	Withdrawn (insufficient funding)	NeuroGeneration
iPSCs	Blood	Evaluate the safety of injecting autologous iPSC-derived dopamine neurons into the brain of PD patients	N/A	USA	1	NCT06422208	2024	2027	Not yet recruiting	Mclean Hospital
AD-MSCs	Subdermal fat deposits	Investigate the safety and efficacy of SCs isolated from autologous microvasculature	N/A	USA	N/A	NCT03297177	2020	2023	Unknown	Healeon Medical, Inc

Since each cell presents with its own advantages and disadvantages, other studies are employing different SC types, one of which is human amniotic epithelial stem cells (hAESCs) [118]. This early phase 1 interventional study, conducted from 2020 to 2023, included 3 participants aged 30 to 70 with primary PD for over 5 years and a Hoehn and Yahr grade of 2 to 4. Participants needed stable medication for over 3 months, effective Levodopa treatment, a stable condition, no contraindications to surgery or anesthesia, and no MRI abnormalities. Each participant received 50 million hAESCs. Similarly, a randomized, double-blind, single-center phase 2 study assessed the efficacy and safety of multiple HB-adMSCs in treating PD [119]. This phase 2 interventional study, conducted from 2021 to 2024, included 24 participants aged 18 to 75. Participants needed a diagnosis of early to moderate Parkinson’s disease for at least 6 months and to have previously banked mesenchymal stem cells from Hope Biosciences [119]. The study had two groups: Group A (15 participants) received HB-adMSCs, and Group B (9 participants) received a placebo. Results, including recorded changes in MDS-UPDRS, CBC, and Neuro-QoL, are posted on ClinicalTrials.gov, but detailed analysis is not yet available.

At the moment, there are seven recruiting clinical trials employing stem cells for treatment of PD, located in the USA, China, and Europe. All these studies try to investigate the safety, tolerability, and efficacy of various SC-based treatment options. For example, one study aims to test the efficacy of human dopaminergic progenitor cell injections for PD patients [120], while another is a Spain-based, multicenter, single-arm, first-in-human, dose-escalation trial investigating the safety and tolerability of intraputaminal transplantation of hESC-derived DA cells for PD [121]. The impact of ESCs is also being studied in a single-center, single-arm, dose-escalation trial with 18 participants in China, exploring the safety, tolerability, and efficacy of hAESCs for idiopathic PD [122]. In addition to ESCs, the next clinical trial is investigating the safety and efficacy of autologous iNSC-DAP in the treatment of PD in China [123], while a US-based phase 1 interventional study with 7 participants is examining the safety and efficacy of the transplantation of human iPSC-derived dopaminergic progenitors [124]. Furthermore, another recruiting, phase 2 study in the US aims to evaluate the efficacy and safety of multiple allogeneic HB-adMSCs for treating PD, with an estimated enrollment of approximately 60 participants [125]. The last still-recruiting study, based in Spain, is an open-labeled pilot study aimed at investigating the effectiveness of a new food supplement containing natural stem cell mobilizers to enhance motor performance and quality of life in PD patients [126]. This study aims to recruit 40 participants.

Although most of the aforementioned clinical trials have not yet reported any results, Kim et al. described the first clinical report on the treatment of 15 PD patients using fetal midbrain precursor cells, a study which took place from 2013 until 2022 [127]. Patients who received medium and high doses of these cells showed significant improvement in motor function without serious complications. The genomic stability of the cells was maintained between passages 5 and 25. This therapy was found to be safe and potentially effective, indicating promising practical applications for future cell-based therapies for PD. In addition to clinical trials, several case studies have reported successful applications of SCT. For instance, a study from Japan involving three PD patients demonstrated that repeated administrations—seventeen in total—of autologous AD-MSCs were both safe and manageable [128]. In another case study, a patient with idiopathic PD received an implantation of autologous induced pluripotent stem cell-derived midbrain dopaminergic progenitor cells, resulting in stabilized or improved symptoms at 18 to 24 months after implantation [129].

Challenges and ethical considerations

Challenges in stem cell therapies

Despite their potential, stem cells face challenges like immunocompatibility [130], even as the FDA approves treatments like Omisirge for blood cancers [131]. Namely, should immune rejection occur, it can lead to graft failure. To overcome this challenge, efforts are being made to improve graft survival by co-transplanting with supportive cell types or neuroprotective agents [132] or employing functionalized composite scaffolds, such as those employed in mouse models of PD [133]. On top of immune rejection, stem cells are also susceptible to instability after long-term expansion and migration to inappropriate regions of the body [134]. An overview of the current stem cell types used in PD and AD treatment, including their advantages and disadvantages, can be found in Table 5.

Table 5.

Overview of stem cells

Stem cell type	Key advantages	Key disadvantages/risks
ESC-derived progenitors	Pluripotent with high differentiation potential	Ethical concerns over ESC origin
	Standardized, scalable manufacturing	Requires immunosuppression (allogeneic)
	Ability to generate pure, defined neuronal subtypes	Risk of tumorigenesis if undifferentiated cells remain
	Consistent quality from a single source	Neurosurgical delivery often required
Allogeneic iPSC-derived progenitors	Can be produced in large, standardized batches	Immunosuppression is often still needed
	Avoids some ethical concerns of ESCs	Manufacturing complexity and long lead times
		Potential genomic instability or tumorigenicity
Autologous iPSC-derived progenitors	Patient-specific, thereby high immune compatibility	Very expensive and time-intensive to manufacture
	Avoids donor matching & long-term immunosuppression	Variable quality across patients
	Eliminates risk of immune rejection	Possible carryover of patient-specific mutations
		Still requires neurosurgery for CNS delivery
Fetal tissue-derived neural cells	Historical evidence of long-term survival and function	Major ethical and logistical limitations
	Naturally patterned developmental profile	Not scalable
		Donor variability affects outcomes
		Risk of graft-induced dyskinesias
MSCs	Broad immunomodulatory and trophic effects	Limited capacity for direct neuronal replacement
	Multiple tissue sources (bone marrow, adipose tissue…)	Variable potency due to donor and/or source
	Can be delivered less invasively (IV, intrathecal)	Short-lived effects
	Good safety record in trials	Poor brain engraftment after systemic delivery
UC-MSCs	Readily available allogeneic source	Often require invasive delivery for CNS targeting
	Lower immunogenicity than adult MSCs	Efficacy evidence still preliminary
	Potential neuroprotective effects	Potential for donor-to-donor variability
MSC-derived EVs/exosomes	Cell-free → no risk of uncontrolled proliferation	Standardization and dosing not established
	Lower immune risk	Limited brain accumulation
	Can cross BBB via intranasal delivery	Manufacturing and quality control challenges
	Easier to store and transport
NSCs	Can integrate into neural circuits	Invasive delivery required
	Secrete neurotrophic factors	Risk of ectopic differentiation or tumorigenesis
	Potential to replace lost neurons and glia	Scalability and purity issues
		No proven efficacy in large human trials yet

Beyond immunocompatibility, challenges in developing effective SCTs include the stemness of stem cells, donor variability, and their differentiation and proliferation capacities. One of the main reasons is that stem cells may lose stemness due to aging or prolonged ex vivo cultivation, reducing their regenerative capacity and treatment effectiveness. Consequently, due to heterogeneity in cell populations and disease presentation, standardization of treatment protocols becomes challenging. To address this issue, researchers must focus on optimizing methods that account for stemness stability through modulating signaling pathways, such as Notch and Wnt/β-catenin [135], or using small molecules to enhance SCs self-renewal capabilities, including glutathione, melatonin, and ascorbic acid (ASA) [136]. While glutathione and melatonin are recognized as antioxidative agents [137], ASA has been shown to increase intracellular HIF1α expression, simulating hypoxic conditions and providing protection against oxidative damage [138]. In addition to small molecules, recent studies have also highlighted the influence of EVs on SCs’ stemness [139]. Notably, ESC-derived EVs containing fibronectin have been shown to play a role in maintaining ESCs’ stemness by activating the integrin-FAK pathway [140]. Furthermore, other studies have validated the effectiveness of electromagnetic stimulation in preserving stemness properties of MSCs [141]. While promising, additional research is needed to elucidate the molecular mechanisms underlying these agents’ actions and assess their clinical viability, especially considering their effects on various stem cell types beyond ESCs and MSCs, which have been the primary focus of current investigations.

After addressing stemness and immunocompatibility concerns, the next critical step in advancing this research toward clinical trials involves maintaining the stem cell’s differentiation capacity, since dysregulated differentiation can lead to ineffective therapy. This approach most commonly involves the application of growth factors (e.g., BDNF, NGF, and GDNF) [142] and epigenetic modifications [143]. Most recently, advancements have also been made in the differentiation of mouse embryonic stem cells (mESCs) toward neuroectoderm and neural progenitor cells [144]. On top of the application of growth factors, other researchers are looking into the application of novel technologies such as photobiomodulation [145] and low-intensity ultrasound [146]. Although various techniques hold promise for promoting SC differentiation, caution is essential, as recent studies reveal that passage number may affect the ability of iPSCs to differentiate into peripheral sensory neurons [147]. This phenomenon should also be studied when working with other cell types.

In addition to selecting the appropriate cell type, SCTs must address challenges related to the purity and viability of isolated cells, as contamination or cell death can undermine treatment effectiveness [148]. For example, NSCs must be carefully extracted and maintained to prevent differentiation before transplantation [149]. Furthermore, determining the optimal route of administration is crucial for effective treatment. Intravenous administration is less invasive but may result in lower cell delivery to the brain, while intracerebral injection ensures direct delivery but is highly invasive [150]. Contrastingly, studies have shown that intrathecal administration can be a balanced approach, providing better cell migration to the central nervous system with reduced invasiveness [151].

Aside from technical challenges, SCT also faces issues with lack of standardized protocols and quality control systems. These, in turn, not only affect the efficacy of the treatment method but also impact the validity and reproducibility of results. Moreover, despite extensive research into the molecular mechanisms underlying the observed benefits of SCT, a consensus on how stem cells precisely contribute to these effects, even in animal models, remains elusive [152]. Arguably, this represents one of the largest challenges to be overcome prior to the widespread application of SCT. Additionally, and to facilitate large-scale application of SCTs, new techniques to facilitate optimized isolation and identification of stem cells must be developed.

Given that the effectiveness of the treatment approach is influenced by factors such as the administered dose and type of stem cells, as shown in studies employing MSCs for the treatment of neurological disorders [153, 154], it is crucial to conduct additional research to determine the optimal treatment parameters. Moreover, further studies are needed to assess the efficacy of different types of stem cell populations used for treatment. Even though SCs boast some comparable characteristics, such as their extensive proliferation and differentiation capacity, they differ in their epigenetic and protein profiles [130]. In addition to investigating disease-specific factors, research should also prioritize the identification of the most suitable stem cell source for treating specific conditions. With that, the disease, dosage, route of administration, and the organ’s physiology must all be considered to select the most optimal cell source.

Besides disease treatment, some technical challenges also remain within the realm of using SCs for disease modeling. While this method could greatly enhance our understanding of certain disease mechanisms and aid in the creation of new treatments, using hiPSCs to study disease pathogenesis comes with several hurdles. One significant issue is the difficulty in modeling diseases with long latency periods, like Alzheimer’s or Parkinson’s. The progression of these diseases in patients may differ significantly from any phenotype that develops in vitro from patient-specific hiPSCs. Another difficulty lies in the inability to effectively replicate diseases in vitro with just one type of specialized cell [155]. Unfortunately, current hiPSC modeling methods fall short in accurately mimicking the interactions between the diseased cell type and other cells within the tissue or the patient’s body. Additionally, studying diseases with significant epigenetic factors using iPSCs is challenging because the reprogramming process often eliminates the epigenetic alterations associated with the disease [156].

Ethical considerations and controversies

While SCT has advanced to clinical trials for various neurological disorders, the field continues to grapple with numerous ethical challenges. A significant issue is the transplantation of ESCs, which requires the use of human fetal tissue [157]. Specifically, in order to obtain fetal ventral mesencephalon (FVM) progenitors that will give rise to dopamine-producing neurons, at least three fetal midbrains are required per grafted side of the brain [158], making this method not only impractical for widespread clinical use but also ethically contentious due to the reliance on aborted tissue [159]. Moreover, although post-mortem studies have confirmed the long-term effectiveness of FVM grafts, with graft-derived DA innervation persisting for up to 24 years, Li et al. report that the clinical benefits of the transplantation diminish after 14 years [160]. Given the limited availability and heterogeneity of this tissue, developing a standardized and unlimited cell source with high efficacy and safety would greatly assist in the commercialization efforts of ESC-based SCTs. One significant advancement towards this new approach is the potential for continuous expansion of human fetal VM cells, a milestone achieved in 2017, although the clinical efficacy of the resulting cell lines remains untested [161].

Interestingly, since embryonic stem cells have been the topic of discussion since the dawn of SCTs, research is also being conducted into the public perception of such treatment approaches to inform treatment design and policymaking. One of these studies, conducted in 2023 at two university hospitals in Sweden, assessed the preference of 498 patients with PD regarding stem cell-based therapies to treat PD in the future [162]. The study found that most patients with PD would accept treatment with hESCs, and that the moral status of embryos did not influence their decisions. The key factors influencing their preferences were treatment effectiveness, type of treatment, previous experience with treatment, and side effects. By emphasizing the need to consider patients’ preferences in developing new treatments, this study sparks crucial discussions about the use of hESCs and can help shape ethical and legal standards for their application. Even though Bywall et al. reported promising findings in their study, the relatively low response rate raises concerns about potential selection bias. Additionally, the exclusion of participants who consistently selected the “standard treatment” option may further limit the generalizability of the results. With that, future research should include open-ended questions to better understand respondents’ choices and qualitative interviews to strengthen the conclusions of the study.

Aside from ethical challenges related to the use of aborted tissue, which are specific for ESCs, an issue shared by all stem cell-based therapies is that of biobanking and treatment availability. More specifically, with a growing number of clinicians suggesting their patients to store umbilical cord blood following birth, concerns regarding potential breaches of privacy and commercialization of health data are on the rise [18]. By collecting samples and associated data from both volunteers and paying participants, biobanks and biomedical research data repositories make their resources accessible to researchers with commercial motivations, such as those developing new treatments. Consequently, one concern is that, while commercial companies might financially benefit from accessing biobank’s resources, the resulting high-cost medicines could make it difficult for individuals to access these therapeutic benefits. As shown in a study by Samuel et al., while biobanks frequently disclose their commercial access agreements, they often lack sufficient detail about the nature of these interactions and the potential risks for both current and future users [163]. Moreover, there is a notable lack of discussion regarding the availability of biobanks for diverse social groups, as predominantly, biobank users are white, middle-class, and well-educated individuals [164]. As a result, this pattern leaves out marginalized communities, who frequently face a higher likelihood of health problems.

Recent advancements and future directions

Bioengineering approaches

Stem cell therapies offer a promising avenue for treating neurological disorders, including neurodegenerative conditions. However, their application has been limited due to challenges in monitoring and directing their differentiation [165]. In response, recent years have seen the emergence of innovative bioengineering approaches aimed at optimizing stem cell delivery, their functionality within target tissues, and their longevity. A key development in this field is the use of nanotechnology. Beyond its proven benefits in cell isolation and purification, research indicates that nanotechnology can also aid in the differentiation of stem cells [166]. Depending on the desired effects, be it optimization of stem cell differentiation or labeling and tracking of implanted cells, this field includes the application of non-toxic and biodegradable collagen [167], carbon [168] and piezoelectric poly (L-lactic acid) (PLLA) nanofibers [169], graphene oxide (GONPs) [170] or superparamagnetic iron oxide nanoparticles (Fe₃O₄) (SPIONs). While the nanofibers and GONPs can be incorporated into scaffolds or matrices and used to assist in the differentiation of MSCs [171], NSCs [172, 173] and iPSCs [174], SPIONs are most commonly employed for labeling and tracking of grafted cells, facilitating their observation using magnetic resonance imaging (MRI) [175, 176].

Besides tracking migration, magnetic nanoparticles (MNPs) can also be used for targeted cell isolation from a multi cell-mixture, also known as magnetic-activated cell sorting (MACS). One of the latest advancements, developed by Wolnik et al. introduces a method to purify hiPSC-derived cardiomyocytes (CMs). Despite significant strides in methodologies for differentiating stem cells into specific cell types, most protocols still produce heterogeneous cell populations. For instance, as demonstrated by Wolnik et al. these populations include CMs, differentiated non-cardiac cell types, and undifferentiated hiPSCs [177]. Nevertheless, these advancements are still paving the way towards creating optimized platforms for disease modeling, drug toxicity screening, and the purification of resulting cells for regenerative applications.

Given that the successful survival and integration of engrafted cells is one of the major challenges in SCT, ongoing research is focused on optimizing stem cell delivery protocols. One such effort involves using PLLA short nanofibers within a thermo-responsive xyloglucan hydrogel, which has demonstrated enhanced engraftment of transplanted DA progenitors in a mouse model of PD [133]. Similarly, Ortega et al. have achieved promising results with an innovative drug delivery system which integrates MSCs and GDNF within a guest–host supramolecular nanoreinforced hydrogel (HG), allowing for their concurrent administration [178]. The developed HG system has shown suitable strength, excellent self-healing properties, good biocompatibility, and the ability to enhance MSC regenerative potential, indicating its potential application in neurodegenerative disorders like PD.

On top of direct delivery of stem cells to the targeted tissue through transplantation, research is also being performed on employing MSC-derived neuron-like cell membrane-coated curcumin PLGA nanoparticles (MM-Cur-NPs) in treating PD mice [179]. After administering MM-Cur-NPs, there was a noticeable decrease in the damage to tyrosine hydroxylase (TH) positive neurons in the dense region of the SN in PD mice. This, in turn, resulted in upregulation in dopamine secretion, regulation of the inflammatory response, reduction of oxidative stress, and improvement in movement disorders.

Since advancements in disease modeling are significantly influencing the development of new treatment approaches, efforts are also being made to optimize 3D in vitro disease models. For instance, Mathes et al. have developed an innovative HG scaffold that replicates the environment found in AD [180]. This scaffold is composed of self-assembling peptides that form β-sheet structures, akin to those in amyloid-β protein aggregates. The team compared the effects of this amyloid-like scaffold on healthy NPCs with those of a natural-mimicking matrix. Their findings revealed that the amyloid-like scaffold induced higher levels of neuroinflammation and apoptosis markers in NPCs. With that, these results provide valuable insights into the impact of amyloid-like structures on NPCs’ phenotypes and behaviors, presenting a promising scaffold for future investigations into AD mechanisms and drug testing. Similar approaches to optimizing 3D in vitro disease models are being shown by Valerio et al. by employing a human platelet lysate (PL) matrix as a 3D extracellular matrix (ECM) alternative [181]. This approach facilitates the growth and specialization of AD-induced iPSC lines into NSCs using the dual-SMAD inhibition method. Specifically, the study demonstrated that the human-derived PL matrix supported the differentiation of iPSCs into NSCs with more human-like characteristics compared to the conventional mouse-derived scaffold, Matrigel. Furthermore, beneficial effects of the application of novel materials in enhancing survival and maturation of iPSCs have also been demonstrated in rat models of PD [182]. Following the transplantation of iPSC-derived DAPs into a neurotrophin-enriched collagen HG, a significant increase in their survival and maturation was observed compared to the control group. Notably, the researchers reported an eightfold improvement in survival and a 16-fold improvement in dopaminergic differentiation, suggesting that supportive hydrogels could significantly enhance the effectiveness of iPSC-derived cell therapy for PD.

Application of electromagnetic fields

Despite the many benefits of SCs, their heterogeneity and somewhat limited availability pose significant challenges. Therefore, creating innovative cell expansion and differentiation techniques is essential for clinical application, including the use of electromagnetic fields (EMFs). When applied on hBM-MSCs, extremely low frequency electromagnetic fields (ELF-EMFs) of 50 Hz have been shown to suppress cell proliferation and induce neural differentiation through regulation of zinc-metallothionein-3 (Zn-MT3) [183]. Specifically, ELF-EMF application significantly downregulated MT3 expression and increased Zn-MT3 complex formation, both essential for maintaining Zn homeostasis during neural differentiation. Similar findings were also reported by Cho et al. wherein hBM-MSCs exposed to ELF-EMFs exhibited increased expression of neuronal cell markers such as MAP2, NEUROD1, NF-L, and Tau [184].

Although most studies employing EMFs tend to do so on stem cells to be transplanted into murine models of spinal cord injury, others are looking into its application for neurodegenerative disorders, including AD and PD. Specifically, a 2022 study by Jadid et al. used 70 Hz ELF-EMFs to pretreat BM-MSCs prior to their transplantation into the left substantia nigra pars compacta (SNpc) of hemiparkinsonian rats [185]. Following SCT, the BM-MSCs exposed to ELF-EMFs exhibited a higher degree of differentiation into neurons and glial cells and decreased cell death, although no difference in locomotor activity was observed between the control and treatment groups. Similar results were reported following the application of 50 Hz pulsating ELF-EMFs on NSCs in vitro, noting increased β−3 tubulin and GFAP immunoreactivity [186]. When coupled with the extensive research confirming that ELF-EMFs promote adult hippocampal neurogenesis through upregulation in NSC proliferation and differentiation [187–190], the promise of this research field is evident. Nevertheless, while EMF-exposed SCs show potential for treating PD and advancing stem cell therapies, further studies are needed to understand the molecular mechanisms and confirm the reproducibility of results.

Aside from the direct application of electromagnetic fields on stem cells in vitro, Yoo et al. reported that EMF stimulation via magnetized AuNPs also significantly improves the efficiency of the reprogramming of somatic cells into induced DA neurons, both in vitro and in vivo, using MPTP- and 6-OHDA-treated mice [191]. In addition to promoting differentiation into dopamine-producing neurons with functional electrophysiological properties, the study also reported observing significant symptom relief, indicating that EMF/AuNP-mediated iDA conversion can alleviate Parkinsonian phenotypes in mouse models of PD. While the results suggest a promising therapeutic strategy for restoring DA neuron function in PD, further research is necessary to understand the long-term effects on the reprogrammed cells.

On top of the promotion of differentiation, other groups are investigating the influence of electrical stimulation on the migration of ventral midbrain derived neural progenitor cells (NPCsvm) from rat fetal brain tissue [192]. Namely, following the application of direct current (DC) electric fields, directional cell migration towards the cathode was noted, regulated by the Wnt/GSK3β signaling pathway [192]. Since this process involves voltage- and time-dependent migration, CLASP2 and tubulin polarization, and is abolished when the Wnt pathway is inhibited, future work is needed to explore the potential of using electrical stimulation with stem cell therapy for neuroregeneration. Similar results were reported by Dhillon et al. wherein Fe2O3-PAA2K-magnetic MNPs and static magnetic fields were used to direct migration and induce neurite outgrowth from the SN to the striatum [193]. Following MNP loading of primary VM and PC12 cells, a well-established model neuronal cell line in PD research, they were transplanted into the SN of an ex vivo rat brain slice model. The behavior of MNPs in magnetic field gradients led to a notable increase in the directional growth of neurites in 2D slices. Although these and other studies in the field successfully directed cell migration using EMFs [194–196], their findings were limited to in vitro and ex vivo settings. Consequently, further research is needed to translate these findings to in vivo environments.

Conclusion

Stem cells are generally categorized based on their source and potency. ESCs, derived from early-stage embryos, can differentiate into any cell type within the body. In contrast, ASCs, such as HSCs or MSCs found in bone marrow, can differentiate into a more limited range of cell types. Due to their limited availability, challenging isolation, and associated ethical issues, researchers sought alternatives to ESCs, HSCs, and MSCs. Consequently, the discovery of iPSCs has revolutionized the field by enabling the reprogramming of adult somatic cells to a pluripotent state, providing a more ethical and versatile alternative to ESCs.

Given their high differentiation and proliferation abilities, stem cells have demonstrated significant promise in various preclinical studies. Over the past 5 years, there has been a notable increase in the application of stem cells for treating a range of neurological disorders, particularly neurodegenerative diseases such as Alzheimer’s and Parkinson’s. These disorders are characterized by the progressive loss of neuronal function and structure, leading to cognitive and motor impairments, most of which have been successfully treated with stem cell transplantation in both in vitro and in vivo studies. Most notably, researchers have reported an increase in DA neurons, levels of dopamine, improvements in motor function, and decreases in apoptosis following treatment.

In addition to preclinical studies, SCT has also advanced to phase 1 and phase 2 clinical trials for patients with AD and PD. This progress is attributed to SCs’ remarkable differentiation and proliferation capabilities, as well as their ability to inhibit inflammation, prevent apoptosis, and stimulate angiogenesis. Over the past 15 years, 76 SCT-based trials have been conducted—27 for AD and 48 for PD. Notably, more than half of these trials have occurred in the last 5 years, including 9 of the 27 SCT trials for Alzheimer’s and 25 of the 48 trials for Parkinson’s. While a few of these trials have shown positive results, the mechanisms underlying the beneficial effects of SCT are still relatively unknown.

Despite some reported successes, due to the relative heterogeneity of isolated SCs, many preclinical studies fail to translate into clinical applications. Additionally, the field faces challenges with inconsistent results across preclinical trials; while some show significant benefits from SCT, these results often cannot be replicated in other studies. These issues are particularly prevalent among BM-MSCs, AD-MSCs, UC-MSCs, and UCB-MSCs, as these are the most frequently utilized cell types in SCT. On top of the challenges in the reproducibility of results, since MSCs are allogeneic, they also have limited neuronal differentiation potential and relatively high rates of immune rejection. Other SC types, while boasting higher reproducibility of results, come with ethical issues, such as the use of aborted tissue for isolating ESCs or fetal tissue to obtain NSCs. A promising solution to these ethical concerns is the use of iPSCs, which can be obtained through the reprogramming of adult cells. However, before iPSCs can advance to clinical trials, several issues need to be addressed, including the correction of mutations, induction of differentiation towards neuronal phenotypes, and ensuring the proliferation and long-term survival of iPSC-derived DA neurons following transplantation.

While iPSCs can mitigate some challenges associated with ESCs, other challenges can be tackled with novel materials and EMFs. Bioengineering approaches can enhance the successful engraftment of transplanted SCs through employment of novel biomaterials in the form of matrices or scaffolds, while EMFs can assist in directing the migration and differentiation of these cells.

Despite these advances, the translational potential of stem cell–derived models remains a subject of ongoing debate. While some researchers champion their use in drug discovery and personalized medicine, others raise concerns about whether current differentiation protocols can faithfully replicate the intricate in vivo environment of the human brain. Resolving these issues will require rigorous methodological standardization and robust cross-laboratory validation. Yet, even in an era of unprecedented scientific collaboration, many of the studies reviewed here exhibit substantial variability in experimental design and treatment approaches—hindering meaningful comparisons and challenging the reproducibility of results.

With that being said, although significant progress has been made in SCTs for AD and PD during the past decade, substantial work remains. Nonetheless, considering the numerous preclinical studies and clinical trials conducted over the past 5 years, SCT demonstrates significant promise for the future treatment of neurodegenerative disorders.

Author contribution

JI conceived the general theoretical framework and made the figures. AA and MK created the tables and analyzed the data. All authors collected the bibliography, wrote the first draft of the article, and developed, refined, and carefully reviewed the final version of the article. All authors have approved the submitted version.

Funding

No funding available.

Data availability

No data was generated for this study.

Declarations

Ethics approval and consent to participate

Not applicable.

Competing interests

The authors declare no competing interests.